1. Field of the Invention
The present invention relates to a photodetector and an optical pickup apparatus.
2. Description of the Related Art
Recently, optical pickup apparatuses are widely used for reproducing or recording information from or in optical discs (CD (Compact Disc), DVD (Digital Versatile Disc), HD DVD (High Definition DVD), Blu-ray Disc®, etc.). To reproduce or record information from or in an information recording layer of the optical discs, the optical pickup apparatuses perform: tracking control based on the differential push-pull method, the three-beam method, etc.; and focus control based on the differential astigmatic method, etc., both using e.g. zero-order light and ±first-order diffracted light generated from laser beams (see, e.g., Japanese Patent Application Laid-Open Publication Nos. 2005-353252 and 2005-346882).
The tracking control based on the differential push-pull method and the focus control based on the differential astigmatic method will be descried with reference to FIG. 28. First, for example, zero-order light and ±first-order diffracted light are generated by a laser beam emitted from semiconductor laser passing through a diffraction grating, etc. The diffraction grating, etc., have diffraction efficiency that generally sets the light amount ratio between zero-order light and ±first-order diffracted light to 10 to 20:1. The zero-order light and ±first-order diffracted light pass through optical systems (polarizing beam splitter, collimator lens, objective lens, etc.) and are condensed onto and reflected by an information recording layer of an optical disc. The reflected light of the zero-order light (hereinafter, referred to as zero-order reflected light) and the reflected light of the ±first-order diffracted light (hereinafter, referred to as ±first-order reflected light) are reflected by the polarizing beam splitter, for example, and received by light-receiving surfaces of photodetectors. That is, as shown in FIG. 28, the zero-order reflected light is received by light-receiving areas A′ to D′ of a light-receiving surface 101 included in the photodetector, the +first-order reflected light is received by light-receiving areas E′ to H′ of a light-receiving surface 102 included in the photodetector, and the −first-order reflected light is received by light-receiving areas I′ to L′ of a light-receiving surface 103 included in the photodetector. Based on photoelectric conversion signals A′ to L′ corresponding to the light amounts of the reflected light of the light-receiving areas A′ to L′, there is performed an operation of: (photoelectric conversion signal A′+photoelectric conversion signal B′)−(photoelectric conversion signal C′+photoelectric conversion signal D′)−k·{(photoelectric conversion signal E′+photoelectric conversion signal F′)−(photoelectric conversion signal G′+photoelectric conversion signal H′)+(photoelectric conversion signal I′+photoelectric conversion signal J′)−(photoelectric conversion signal K′+photoelectric conversion signal L′)} (k: light amount of zero-order light/light amount of ±first-order diffracted light), thereby to detect a good tracking error signal that offsets due to lens shifts, inclination of the optical disc, etc. are removed from. There is also performed an operation of: (photoelectric conversion signal A′+photoelectric conversion signal C′)−(photoelectric conversion signal B′+photoelectric conversion signal D′)−k·{(photoelectric conversion signal E′+photoelectric conversion signal G′)−(photoelectric conversion signal F′+photoelectric conversion signal H′)+(photoelectric conversion signal I′+photoelectric conversion signal K′)−(photoelectric conversion signal J′+photoelectric conversion signal L′)}, thereby to detect a good focus error signal that leaking-in of the tracking error signal when crossing tracks, etc. is reduced from. Based on the tracking error signal and the focus error signal, the zero-order light can be focused on the information recording layer of the optical disc and driven to follow the track thereof, by moving the objective lens in the radial direction and the optical axis directions of the optical disc. As a result, information can favorably be reproduced or recorded from or in the optical disc.
In recent years, there are realized so-called multilayer optical discs with a plurality of information recording layers to be allowed to record more information, as compared to optical discs with only one information recording layer. The tracking control and the focus control are also necessary for the information recording layers of the multilayer optical discs to favorably reproduce or record information.
However, when the tracking control and the focus control are performed to reproduce or record information from or in one information recording layer in a multilayer optical disc, zero-order light (so-called, stray light) is generated from another information recording layer different from the one information recording layer and a tracking error signal and a focus error signal may not be detected precisely. Therefore, it may become difficult to perform good tracking control and focus control, and precise information reproduction or recording may become difficult or impossible.
With reference to FIGS. 29A, 29B, and 30, there will be described effects of stray light for reproduction and recording of information of an optical disc 100 including two information recording layers L0 and L1, for example. The information recording layer L1 of the optical disc 100 is made of a translucent reflective film to enable reproduction or recording of information for either of the information recording layers L0 and L1. Therefore, as shown in FIG. 29A, when zero-order light is condensed on the information recording layer L0, there is generated not only zero-order reflected light from the information recording layer L0 (solid line) but also zero-order reflected light from the information recording layer L1 (broken line). As shown in FIG. 29B, when zero-order light is condensed on the information recording layer L1, there is generated not only zero-order reflected light from the information recording layer L1 (solid line) but also zero-order reflected light that is obtained from zero-order light having passed through the information recording layer L1 and being reflected from the information recording layer L0 (broken line). As a result, the light-receiving surfaces 101 to 103 of the photodetector receive not only the zero-order reflected light and the ±first-order reflected light (solid lines) to be received from one information recording layer, but also the zero-order reflected light from another information layer as shown by a shaded portion within a broken line in FIG. 30. Therefore, the tracking error signal and the focus error signal based on the photoelectric conversion signals A′ to L′ from the light-receiving areas A′ to L′ become signals including a component of the zero-order reflected light from another information layer. In accordance with the zero-order reflected light from another information layer, the tracking error signal and the focus error signal may become signals having offsets and amplitude variations generated in the tracking error signal and the focus error signal not including the component of the zero-order reflected light from another information layer That is, the detected tracking error and focus error signals may not accurately reflect the displacement of the zero-order light for the one information recording layer of the optical disc 100. Especially, since the photoelectric conversion signals E′ to L′ from the light-receiving areas E′ to L′ to receive only the ±first-order reflected light are amplified (by k-times) to correct a light amount ratio between the zero-order light and the ±first-order diffracted light, the effect of the zero-order reflected light from another information recording layer may be increased and more inaccurate tracking error signal and focus error signal may be detected. As a result, it may become difficult to perform good tracking control and focus control, and precise information reproduction or recording may become difficult or impossible.
Therefore, conventional technologies shown in the above patent documents 1, 2, etc., have proposed a method of disposing a light-receiving surface for canceling out the zero-order reflected light from another information recording layer received by the light-receiving surfaces 102 and 103 separately from the light-receiving surfaces 102 and 103 for receiving the first-order diffracted light. However, in these conventional technologies, the light-receiving surface must newly be disposed for canceling out the zero-order reflected light from another information layer, which may cause increases in the number of processes, cost, complexity, size etc., of the photodetector.